Method and device for optoelectronic distance measurement

ABSTRACT

The invention is directed to a method for optoelectronic distance measurement and a measurement device based on this method, in which two light emitters, particularly laser emitters and two photodiode receivers are used for calibration. Part of the modulated output of the main emitter reaches the measurement object and then arrives at the main photoreceiver in the form of scattered light and another part of the output of the main emitter travels directly to a reference photoreceiver, while a part of the modulated output of the reference light emitter is guided directly to the main photoreceiver and another part is guided directly to the reference photoreceiver. According to the invention, the light intensities of the main emitter and the reference emitter are simultaneously modulated with different modulation frequencies (f 1 , f 2 ), and a signal mixture is formed in the main receiver as well as in the reference receiver, which signal mixture contains a signal with the intensity modulation frequency (f 1 ) of the main emitter and a signal with the intensity modulation frequency (f 2 ) of the reference emitter. The phases of the two signals of the signal mixture are measured simultaneously and the separation of the two phases is carried out by the different frequencies in a subordinate intermediate frequency range and by the different intensity modulation frequencies. At least two sequential measurement processes are preferably carried out, wherein the intensity modulation frequencies of the reference emitter and main emitter are exchanged in the second measurement process. Phase errors depending upon temperature, aging and reception power can be entirely eliminated by the invention in electro-optic distance measuring devices.

FIELD OF THE INVENTION

[0001] The invention is directed to a device and method foroptoelectronic distance measurement, wherein an intensity-modulated mainlight beam emitted by a main light emitter is directed to a measurementobject at a distance, wherein the distance (D₀) of a measurement objectfrom an observation point is to be measured, and a light scattered atthe observation point reaches a main photoreceiver via reception optics,and a branched part of the main light beam is simultaneously directed toa reference photoreceiver via a first known reference distance (D₁), anintensity-modulated reference light beam emitted by a reference lightemitter reaches the reference photoreceiver via a second known referencedistance (D₂), and a part of the reference light beam reaches the mainphotoreceiver via a third reference distance (D₃), and wherein thesignals delivered by the main photoreceiver and the referencephotoreceiver are subjected to a comparative signal evaluation forobtaining a corrected measurement signal, wherein the light intensitiesof the main emitter and the reference emitter are simultaneouslyintensity-modulated at different frequencies, wherein the signalmixtures supplied by the main receiver and the reference receiver, eachof the signal mixtures contains a signal component with the intensitymodulation frequency of the main emitter and a signal component with theintensity modulation frequency of the reference receiver, are convertedto an intermediate frequency range containing two frequency components,wherein one frequency component is formed by the signal of the referenceemitter and the other frequency component is formed by the signal of themain emitter, and the separation of the phase information contained inthe two simultaneously occurring intermediate frequency signals is basedon the different frequencies in the intermediate frequency range and thedifferent modulation frequencies for the intensity modulation of themain light beam and reference light beam for purposes of comparativesignal evaluation.

BACKGROUND OF THE INVENTION

[0002] Optoelectronic measurements of distances up to 100 m withaccuracy within a few millimeters have gained importance for numerousapplications, particularly in the construction and the plant engineeringindustries. Such distance measurement systems should be as dynamic aspossible making it possible to process very weak signals as well asstrong signals. Such measurement systems render superfluous the use ofdefined target marks on the object whose distance from an observationpoint is to be determined. The possibility of direct distancemeasurement at determined surfaces, i.e., without the use of targetmarks, makes possible reduced manufacturing times and cost savingsaccompanied by lower manufacturing tolerances, particularly in theindustries mentioned above.

[0003] Processes and devices for accurate optoelectronic distancemeasurement are known. In most cases, as well as in the case of theinvention, a preferably sine-shaped intensity-modulated beam from alight source, particularly a laser diode, is directed onto an object tobe measured. The intensity-modulated light, which is backscattered bythis measurement object, is detected by a photodiode. The distance to bemeasured is given by the phase shift of the sinusoidal modulated lightintensity backscattered from the measurement object in relation to theemitted light intensity from the light source.

[0004] A principal difficulty in high-precision distance and phasemeasurement systems of the type mentioned above is the elimination oftemperature-dependent and aging-dependent parasitic phase changes in thelight source, that is, particularly in the laser diode transmitter andin the photodiode receiver. Various methods are known for counteringthis difficulty.

[0005] One possibility, described in EP 0 701 702 B1, is to use amechanically switchable reference distance. In this case, anintensity-modulated laser beam is initially directed, in a firstmeasurement, to the measurement object and then, in a second referencedistance measurement, is guided directly to the photoreceiver via atiltable mirror. The influences of temperature and aging on thestructural component parts are eliminated by subtracting the measuredphases. However, since widely varying optical reception power must beexpected with alternating distance and reference distance measurement, ameasurement error arising in this way is not eliminated. Anothersubstantial disadvantage of this concept is the use of moving mechanicalcomponents, which limit the reliability and service life of the entiremeasurement system.

[0006] Other known distance measurement devices of the type underdiscussion, which are described in DE 196 43 287 A1, work with areference photoreceiver and a main photoreceiver. In this case, a partof the intensity-modulated laser light is directed to the measurementobject and then to the main photoreceiver and another part which isdivided from the laser light beam is guided directly to the referencephotoreceiver. No moving mechanical switch is required since thereference photoreceiver is constantly illuminated during measurement.However, in this concept, while the phase response of the laser diodetransmitter is eliminated, the phase behavior of the receptioncomponents which change over time and are generally different for themeasurement arm and reference measurement arm are not eliminated.Further, in distance measurement devices of this type, sharply differentreception power in the two arms resulting in further phase errors mustalso be taken into account.

[0007] In another known optoelectronic distance measurement device (seeU.S. Pat. No. 4,403,857), two laser emitters and two photodiodereceivers are used to eliminate the above-mentioned phase errors. Inthis device, a portion of the intensity-modulated output of a main lightemitter is guided directly onto the measurement object, from which itarrives at the main photoreceiver as scattered light. Another portion ofthis transmitted output is guided via an exact known first referencedistance to a reference photoreceiver. Further, there is a referencelight emitter whose delivered output is likewise intensity-modulated anda portion of which reaches the main photoreceiver via a second referencedistance, while another portion is guided directly to the referencephotoreceiver via a third reference distance.

[0008] The main light emitter and the reference light emitter areactivated successively via an electronic changeover switch. Thismeasurement principle requires no mechanical changeover switch. Inaddition, phase changes caused by temperature and aging are completelyeliminated in the transmitting unit as well as in the reception unit.However, since substantial differences in reception power must be takeninto account in measurements with the signals of the main light emitterand reference light emitter, the phase errors resulting from this arealso not eliminated in the concept upon which this known distancemeasurement device is based. Phase errors depending on reception powerare particularly noticeable with avalanche photodiodes (APD) which arepreferred as main receivers because of other advantages. At highamplifications, saturation effects gradually come about as outputincreases, so that avalanche gain is dependent on the received output.Therefore, there occurs, in addition, an output-dependent phase rotationin the case of reception of high-frequency-modulated optic radiation.Further, the generated charge in the barrier layer of the APD varieswith the reception power, so that barrier layer distance as well asbarrier layer capacity are influenced. As the barrier layer capacitychanges, so does the phase behavior of the low-pass formed by it. Withhigh APD gain factors, a phase rotation of greater than 5°, as a rule,can be brought about in this way with a reception power variation of twoorders of magnitude.

SUMMARY OF THE INVENTION

[0009] It is thus the object of the invention to provide a distancemeasurement method and a device operating by this method by which ahighly accurate distance measurement can be achieved and which iscompletely independent from phase errors depending on temperature, agingand reception power. Mechanical or electronic changeover switches aredispensed with and the total measuring time for obtaining reliablemeasurement results is appreciably reduced.

[0010] In a method for optoelectronic distance measurement according tothe invention, the invention is characterized in that the lightintensities of the main emitter and reference emitter are modulatedsimultaneously at different frequencies, wherein the signal mixturessupplied by the main receiver and reference receiver, each of whichsignal mixtures contains a signal component with the intensitymodulation frequency of the main emitter as well as a signal componentwith the intensity modulation frequency of the reference emitter, areconverted to an intermediate frequency range containing two frequencycomponents, wherein one frequency component is formed by the signal ofthe reference emitter and the other frequency component is formed by thesignal of the main emitter, and in that the separation of the phaseinformation contained in the two simultaneously occurring intermediatefrequency signals is carried out based on the different frequencies inthe intermediate frequency range and the different modulation frequencyfor the intensity modulation of the main beam and reference beam forpurposes of comparative signal evaluation.

[0011] Advantageous further developments of this distance measurementmethod according to the invention are defined in the dependent patentclaims.

[0012] A device according to the invention for optoelectronic distancemeasurement with the features of the invention is characterized,according to the invention, by a device by which the light beams emittedby the main emitter and by the reference emitter can beintensity-modulated simultaneously with different frequencies in eachinstance.

[0013] Advantageous constructions of this distance measurement deviceaccording to the invention are likewise defined in further dependentpatent claims.

[0014] In a manner similar to the distance measurement process describedin U.S. Pat. No. 4,403,857, two light transmitters, particularly lasers,and two photodiode receivers are used in the present invention. However,according to the invention, in contrast to this known method, the lightof the first light transmitter, designated as main emitter, which ispreferably sinusoidal intensity-modulated by a first modulationfrequency f₁, is directed to the surface of a measurement object. Thelight which is backscattered from the measurement object and which islikewise intensity-modulated reaches the second photoreceiver,designated as main receiver, for example, via reception optics. At thesame time, a portion of the modulated light of the main emitter isguided directly via a first reference distance to the secondphotoreceiver, designated as reference receiver. The reference emitteris intensity-modulated, likewise preferably in sinusoidal manner, with asecond modulation frequency. A portion of its modulated optical beamreaches the main receiver via a second, known reference distance andparticularly via a scattering medium, while another component of itsmodulated optical beam arrives at the reference receiver via a thirdreference distance.

[0015] The two receivers are simultaneously acted upon by the twoemitter signals, so that, in contrast to the distance measurement methoddescribed in U.S. Pat. No. 4,403,857, no changeover switch is requiredand the measurement time is appreciably reduced. The photoreceiversconvert the detected modulated optical outputs into photocurrents, whichare subsequently converted into voltages, preferably, by transimpedanceamplifiers.

[0016] The two signal voltages obtained in this manner are subsequentlyconverted by associated mixers into suitable intermediate frequencyranges using a locally generated frequency and are then evaluated afteranalog-to-digital conversion of a signal evaluation for error-freedetermination of the phase shift caused by the signal rise time ortransit time and accordingly for determining the distance.

BRIEF DESCRIPTION OF DRAWINGS

[0017] The invention and advantageous details are explained more fullyin the following with reference to the drawings.

[0018]FIG. 1 shows a schematic view of a preferred construction of adistance measurement device used to practice a method in accordance withthe invention;

[0019]FIG. 2a shows a graphical representation of an intermediatefrequency signal mixture obtained at the output of amplifier 16, of FIG.1, in the time domain; and

[0020]FIG. 2a shows a graphical representation of an intermediatefrequency signal mixture obtained at the output of amplifier 16, of FIG.1, in the frequency domain.

DETAILED DESCRIPTION OF THE INVENTION

[0021] The basic arrangement of a distance measurement device operatingby the method according to the invention contains a first lighttransmitter, particularly a laser, designated as main emitter 1, asecond light transmitter, particularly a laser, designated as referenceemitter 2, a first photoreceiver, designated as reference receiver 4,and a second photoreceiver, designated as main receiver 5. The mainemitter 1 can be a comparatively high-power edge-emitting laser diode(EEL) whose emission wavelength is, e.g., λ₁=650 nm, as is indicated inthe drawing. A laser diode which preferably radiates at a differentwavelength, for example, λ₂=850 nm, e.g., a VCSEL (Vertical CavityEmitting Laser Diode), is used as a reference emitter 2. The choice ofdifferent wavelengths for the main emitter and reference emitter makeoptic filtering possible, explained in more detail below, so thatproblems due to optical crosstalk can be reduced. A PIN photodiode ispreferably used as reference receiver 4, while an avalanche photodiodeis preferably provided as main receiver 5.

[0022] The idea of modulating the intensity of the radiation outputemitted by the main emitter 1 and reference emitter 2, respectively, ata determined measurement time with different frequencies, preferablysinusoidal, has decisive importance for the invention. Accordingly, forexample, the main emitter 1 is intensity-modulated (initially) atfrequency f₁ and the reference emitter 2 is intensity-modulated(initially) at frequency f₂. The two modulation frequencies f₁, f₂ areobtained via two frequency synthesizers 21 and 22 controlled by a commonoscillator 20. The modulation signals with frequencies f₁ and f₂,respectively, are supplied to an exciter circuit (not shown) for themain emitter 1 and reference emitter 2, e.g., by high-frequencyamplifiers 23 and 24, respectively.

[0023] The bundled main light beam 30 which is emitted by the mainemitter 1 and, for example, guided in a tube 40, first reaches a beamsplitter 3, which splits the main light beam 30 into two components,namely, a main component 32 directed to the measurement object, and abranched component 33, which reaches the reference receiver 4 via afirst known reference distance D₁ with the intermediary of a diffuser 51or scatterer. A homogeneous modulation phase distribution in the beamcross section before the reference receiver 4 is ensured by the diffuser51 or scatterer. A semitransparent mirror, a prism, a glass plate, anintegrated-optic beam splitter, a diffractive element, e.g., a hologramor the like, can be provided as beam splitter 3. However, it should beemphasized that the beams of the reference distances need notnecessarily be collimated or deflected by beam splitters. Alternatively,for example, volume scattering or direct illumination of the receiver,e.g., by means of a diffuser, can be provided. The component 36 of themeasurement beam 32 which is backscattered from the measurement objectlocated at a distance arrives at the main receiver 5 via collectingoptics 37. The reference light beam 31 which is emitted by the referenceemitter 2 and which is generally collimated is split into two componentsby means of a beam splitter 6, wherein a first component 34 arrives atthe reference receiver 4 via a second reference distance D₂ and adiffuser or the diffuser 51 or scatterer, while a second component 35 isinitially applied, via a third reference distance D₃, preferably via anoptic filter 41 tuned to the wavelength of the reference emitter, to ascattering medium (scatterer) 11 and then, as scattered component,together with the component 36 of the main light beam that isbackscattered from the measurement object, is applied to the mainreceiver 5. In principle, any scattering material can be used asscatterer 11. Even the housing wall would be suitable. However, in orderto monitor the scatter light output of the reference emitter light, thedegree of scatter of the scattering material should be adapted to thereception system. Since an intensive output damping of the referenceemitter beam 35 is aimed for in view of the generally weak measurementsignals, a scattering material with a low degree of scatter, e.g., blackpaper, black velvet or the like, is advantageous at least with respectto the main emitter 5.

[0024] The use of the scatterer 11, which is very advantageous inconnection with the invention, is based on the following observation:The modulation phase is not homogeneous in the beam cross section oflaser diodes, i.e., different points of the beam cross section havedifferent phases with respect to the modulated light intensity. Withhomogeneous backscattering, a phase averaged over the beam cross sectionis measured. However, in the event that determined regions of the beamare cut out or masked by the surface of the measurement object, e.g.,when one portion of the light spot impinges on black, absorbent regionsand another portion impinges on white, highly backscattering regions ofthe surface of the measurement object, the average phase changes and ameasurement error occurs depending on the unknown surface of themeasurement object. This error cannot be compensated. In most cases,however, all points of the light spot are backscattered with virtuallythe same intensity.

[0025] If a reference emitter beam 35 is guided directly to the mainreceiver 5, only a small portion of the beam cross section would bedetected because of the small APD surface. The phase of the beammeasured in this way generally does not represent the average modulationphase. In addition, the phase distribution in the beam cross section isnot constant over time and, moreover, depends on the temperature. Byusing the scatterer 11, it is ensured that signal components aredetected from all points of the beam cross section. Therefore, anaverage phase is measured which has a more constant behavior than apoint phase. Errors occurring due to point measurement of the phase ofthe reference emitter beam 35 are reduced by the scatterer 11.

[0026] In order to prevent such phase errors, it may also beadvantageous to guide the beams 33 and 34 via a scatterer, e.g., thediffuser 51, before the modulated light is detected by the referencereceiver 4.

[0027] It is another object of the scatterer 11 to effect a strongoutput damping of the reference emitter light. Because of the very weakmeasurement signals, the reception system is very sensitive. The outputdamping prevents overloading of the APD in the main receiver 5.Moreover, the shot noise which increases with the detected light outputis reduced in this way.

[0028] It is important to adapt the scatter output of the referenceemitter light to the system so that an optimal signal-to-noise ratio(SNR) is achieved. If the scatter light output is too high, extensiveshot noise must be taken into account and, therefore, with weakmeasurement signals, a poor SNR should be expected when determining thephase of the measurement signal. On the other hand, when scatter lightoutputs are too low, the SNR is poor when determining the phase of thereference emitter signal. Since this phase is also included in thedistance measurement, a measurement error can result. Thus, it isadvisable to find a suitable compromise. For a distance measurementrange of up to 100 m, tests have shown that the detected scatter lightoutput of the reference emitter beam 35 should be approximately equal toan output of the measurement beam 36 which is backscattered from adistance of 15 m. In this instance, a backscattering coefficient of themeasurement object surface of 0.5 and homogeneous backscattering areassumed. At an output of the measurement beam 32 of 1 mW and a diameterof the reception optics 37 of 50 mm, this corresponds to a detectedscatter light output of the reference emitter beam of 0.7 nW.

[0029] The photocurrents supplied by the reference receiver 4 and mainreceiver 5 are initially converted into corresponding measurementvoltages x₂ and x₁ via transimpedance amplifiers 9 and 10.

[0030] In principle, impedance-monitored (50-Ohm) HF-output amplifierscan be used instead of the transimpedance amplifiers. However, as arule, such impedance-monitored HF-output amplifiers have inferior noisecharacteristics and lower gain. It may be useful, however, at least forcost reasons, to employ an impedance-monitored HF-output amplifier forthe reference arm instead of the transimpedance amplifier 9, wherein astrong signal can be achieved with lower current consumption.

[0031] The signal x₂ originating from the reference receiver 4 is thenapplied to a measurement signal input of a first mixer 7, while signalx₁ arrives at the measurement signal input of a second mixer 8. The twomixers 7 and 8 are controlled by the same local frequency oscillator 20(mother oscillator) via frequency synthesizer 12 with a frequency f_(LO)which is selected in such a way that there is a signal mixture withfrequency components f_(ZF1) and f_(ZF2) also in the intermediatefrequency range occurring on the output side. In this case, it isimportant that the modulation frequencies of the main emitter 1 andreference emitter 2 are different and that the main receiver 5 andreference receiver 4 both supply a signal mixture comprising two signalswith frequencies f₁ and f₂. This signal mixture is converted in bothsignal arms with, as a rule, identically constructed mixers 7 and 8 andthe local oscillator signal of the frequency f_(LO) in theabove-mentioned intermediate frequency range. Also, direct mixing viaavalanche photodiodes (APD) is possible in this connection, wherein thesignal of the local oscillator 12 is superimposed directly on the APDoperating high voltage. This local oscillator signal is mixed with thereception signal by the resulting modulation of the avalanche gain, sothat the APD output current contains both intermediate frequency signalcomponents f_(ZF1) and f_(ZF2). The high-frequency amplifier andhigh-frequency mixer can accordingly be dispensed with. In this case,however, avalanche photodiodes are to be used for the reference arm aswell as for the measurement arm. The first intermediate frequency signalwith frequency f_(ZF1) is subsequently derived from the reception signalwith the first modulation frequency f₁ and the intermediate frequencysignal with the second intermediate frequency f_(ZF2) is derived fromthe reception signal with the second modulation frequency f₂. Afterlow-pass filtering 13 and 14 for eliminating the signal components withsum frequencies f₁+f_(LO) and f₂+f_(LO) and for noise signal reductionand gain 15 and 16, the suitably amplified intermediate frequencysignals x₄ and x₃ are sampled by analog-to-digital converters 17 and 18.The first and second intermediate frequencies f_(ZF1) and f_(ZF2) andthe sampling time of the analog-to-digital converters 17, 18, that is,the measurement window, are advisably selected in such a way that anintegral number of cycles of the two intermediate frequency signalsf_(ZF1), f_(ZF2) lie in the measurement window of the digital sampling.This selection prevents a leakage effect, as it is called, which occursin the digital, discrete Fourier transform (DFT) when the frequencycomponents do not lie in the frequency raster of the DFT; e.g., 40 kHzand 60 kHz for a 1-kHz frequency raster, i.e., the spacing of thediscrete frequency values is 1 kHz and the associated measurement windowis 1/1 kHz=1 ms.

[0032] A digital Fourier transform 19 of the sampled signal mixture x₃of the main receiver arm 42 and, independent from this, of the sampledsignal mixture x₄ of the reference receiver arm 43, e.g., for adetermined measurement window as indicated above, supplies the phases ofthe light signal components 33 and 36 of the main emitter 1, the phasesof the light signal components 34 and 35 of the reference emitter 2, andphase shifts which are dependent upon temperature, aging, and receptionpower and which are brought about in the main receiver arm and referencereceiver arm. Accordingly, four phase relationships are determined.

[0033] Since the phase can only be definitely measured in an interval of0 to 2_(π), but the measurement distance produces substantially greaterphase shifts in most cases, the modulation frequencies are changed in asecond measurement pass, i.e., the main emitter 1 is now sinusoidalintensity-modulated with frequency f₂ and the reference emitter 2 issinusoidal intensity-modulated with frequency f₁, according to asignificant improvement, in accordance with the invention for achievinga definite distance measurement and for improving the measurementresults. The measurement process described above is repeated with thesenew settings so that four further phases result for the sampled signalmixtures in the main receiver arm and reference receiver arm for thesenew settings.

[0034] Since the signal of the main emitter 1 and the signal of thereference emitter 2 in the arm of the main receiver 5 and in the arm ofthe reference receiver 4 traverse the same components, as was shown, thephase behavior of the respective receiver components is completelyeliminated by phase subtraction with respect to the signal phasesmeasured in the main reception arm 42 and in the reference reception arm43. Moreover, the reception ratios are constant, since the main emittersignals and reference emitter signals simultaneously pass the respectivereception arms. As was also shown, the separation of the signals iscarried out via the different modulation frequencies f₁ and f₂. Phaseerrors depending on reception power are accordingly likewise eliminated.

[0035] By additional subtraction of phase differences, the phasebehavior of the main emitter 1 and the phase behavior of the referenceemitter 2 are eliminated in addition, so that there finally remains onlya constant phase difference which is formed by the indicated pathdifferences of reference distances D₁, D₂ and D₃ internal to the deviceand by the measurement distance D₀ (not shown in more detail in FIG. 1)of the two emitter signals outside the two laser emitters. Themeasurement distance D₀ influences the phase of the modulated lightcomponent 32 of the main emitter 1 detected by the main receiver 5,reference distance D₁ influences the phase of the modulated lightcomponent 33 of the main emitter 1 detected by the reference receiver,reference distance D₂ influences the phase of the modulated lightcomponent 34 of the reference emitter 2 detected by the referencereceiver, and reference distance D₃ influences the phase of themodulated light component 35 of the reference emitter 2 detected by thereference receiver. Since the transit times internal to the device areknown via reference distances D₁, D₂ and D₃ outside the emitters 1, 2and are constant, the distance to be measured can be determined. Thedistances are determined in accordance with the method of the inventioncompletely independently from the phase behavior of the transmittingunits and reception units.

[0036] Phase difference averaging by means of Fourier transformation isexplained in the following section. The basic shape of the intermediatefrequency signals, which are sampled by the analog-to-digital converters17 and 18, is shown in FIG. 2a. The signal mixture in the frequencydomain is also shown in FIG. 2b.

[0037] Initially, the main emitter 1 emits radiation with modulationfrequency f₁ and the reference emitter 2 emits radiation with modulationfrequency f₂. Each mixer input signal x₁ and x₂ has two sine-shapedsignal components with frequencies f₁ and f₂ of modulation.

[0038] Let:

x ₁ ={circumflex over (x)} _(1,1) cos(2f ₁ t+ψ _(HS)(f ₁)+ψ_(HE)(f ₁)−2f₁2D ₀ /c)+ {circumflex over (x)} _(1,2) cos(2f ₂ t+ψ _(RS)(f ₂)+ψ_(HE)(f₂)−2f ₂ D ₃ /c)  (1)

x ₂ ={circumflex over (x)} _(2,1) cos(2f ₁ t+ψ _(HS)(f ₁)+ψ_(RE)(f ₁)−2f₁ D ₂ /c)+ {circumflex over (x)} _(2,2) cos(2f ₂t+ψ_(RS)(f ₂)+ψ_(RE)(f₁)−2f ₂ D ₂ /c),  (2)

[0039] where

[0040]_(ψHS)(f₁) is the sum of the initial phase of the synthesizer (21)and phase shift of the driver (23) and main emitter (1) at frequency f₁;temperature-dependent and aging-dependent;

[0041] ψ_(RS)(f₂) is the sum of the initial phase of the synthesizer(22) and phase shift of the driver (24) and reference emitter (2) atfrequency _(f2); temperature-dependent and aging-dependent;

[0042] ψ_(HE)(f₁) is the sum of the phase shift of the main receiver (5)and transimpedance amplifier (10) at frequency f₁; temperature-dependentand aging-dependent;

[0043] ψ_(HE)(f₂) is the sum of the phase shift of the main receiver (5)and transimpedance amplifier (10) at frequency f₂; temperature-dependentand aging-dependent;

[0044] ψ_(RE)(f₁) is the sum of the phase shift of the referencereceiver (4) and transimpedance amplifier (9) at frequency f₁;temperature-dependent and aging-dependent;

[0045] ψ_(RE)(f₂) is the sum of the phase shift of the referencereceiver (4) and transimpedance amplifier (9) at frequency f₂;temperature-dependent and aging-dependent;

[0046] D₀ is the measurement distance;

[0047] D₁ is the constant and known distance internal to the device;

[0048] D₂ is the constant and known distance internal to the device;

[0049] D₃ is the constant and known distance internal to the device;

[0050] c is the light velocity in air.

[0051] By mixing (multiplying) the signals from equations (1) and (2)with the local oscillator signal of frequency f_(LO) and subsequentlow-pass filtration,

x ₃ ={circumflex over (x)} _(3,1) cos(2f _(ZF1) t+ψ _(HS)(f ₁)+ψ_(HE)(f₁)+ψ_(ZF3)(f _(ZF1))−2f ₁2D ₀ /c)+ {circumflex over (x)} _(3,2) cos(2f_(ZF2) t+ψ _(RS)(f ₂)+ψ_(HE)(f ₂)+ψ_(ZF3)(f _(ZF2))−2f ₂ D ₃ /c)  (3)

x ₄ ={circumflex over (x)} _(4,1) cos(2f _(ZF1) t+ψ _(HS)(f ₁)+ψ_(RE)(f₁)+ψ_(ZR4)(f _(ZF1))−2f ₁ D ₁ /c)+ {circumflex over (x)} _(2,2) cos(2f_(ZF2) t+ψ _(RS)(f ₂)+ψ_(RE)(f ₂)+ψ_(ZF4)(f _(ZF2))−2f ₂ D ₂ /c)  (4)

[0052] with the intermediate frequencies

f _(ZF1) =|f ₁ −f _(LO)|  (5)

and

f _(ZF2) =|f ₂ −f _(LO)|  (6).

[0053] By means of the low-pass filtration, the signal components withsum frequencies f₁+f_(LO) and f₂+f_(LO), which are also formed by thenonlinear mixing process, are eliminated. The noise level is alsoreduced in this way.

[0054] Let

[0055]_(ψZF3)(f_(ZF1)) be the sum of the initial phase of thesynthesizer (12), initial sampling phase of the ADC (18) and phase shiftof the low-pass (14) and amplifier (16) at frequency f_(ZF1),

[0056] ψ_(ZF3)(f_(ZF2)) be the sum of the initial phase of thesynthesizer (12), initial sampling phase of the ADC (18) and phase shiftof the low-pass (14) and amplifier (16) at frequency f_(ZF2);

[0057] ψ_(ZF4)(f_(ZF1)) be the sum of the initial phase of thesynthesizer (12), initial sampling phase of the ADC (17) and phase shiftof the low-pass (13) and amplifier (15) at frequency f_(ZF1);

[0058] ψ_(ZF4)(f_(ZF2)) be the sum of the initial phase of thesynthesizer (12), initial sampling phase of the ADC (17) and phase shiftof the low-pass (13) and amplifier (15) at frequency f_(ZF2).

[0059] The intermediate frequency signals x₃ and x₄ are sampledsynchronously by the A-D converters 17, 18. The signal components withthe different intermediate frequencies f_(ZF1) and f_(ZF2) can beseparated by discrete Fourier transformation of the sampled signals x₃and x₄ in block 19.

[0060] In FIG. 2b, a signal mixture x₃ is shown by way of example in thetime domain with frequency components f_(ZF1)=40 kHz and f_(ZF2)=60 kHz.At right, the amount of the Fourier-transformed signal mixture isplotted over the intermediate frequency f_(ZF). Two sharp signal peaksare shown in the respective intermediate frequencies. At otherfrequencies, the values of the spectrum are virtually zero. Thedifferent levels of the peaks result from different amplitudes of thesignal components in the time domain (0.7 V, 0.4 V).

[0061] The values of the transformed signal mixture in the frequencydomain are complex, i.e., it is composed of a real component and animaginary component according to

X ₃(f _(ZF))=Re{X ₃(f _(ZF))}+j·lm{X ₃(f _(ZF))}.

[0062]FIG. 2 shows the following amount:

_(—) X ₃(f _(ZF))_(—) ={square root}{square root over(lm²{X₃(f_(ZF))}+Re²{X₃(f_(ZF))})}.

[0063] The phases of the separated signal components at the intermediatefrequencies f_(ZF1) and f_(ZF2) in question can be determined from thecomplex values at the respective frequencies f_(ZF1) and f_(ZF2) bymeans of the arctan function according to

ψ₁(f ₁)=arctan(lm{X ₃(f _(ZF1))}/Re{X ₃(f _(ZF1))})

and

_(ψ2)(f ₂)=arctan(lm{X ₃(f _(ZF2))}/Re{X ₃(f _(ZF2))}).

[0064] For x₃ from equation (3), they are

_(ψ1)(f ₁)=ψ_(HS)(f ₁)+ψ_(HE)(f ₁)+ψ_(ZF3)(f _(ZF1))−2f ₁2D ₀ /c  (7)

and

ψ₂(f ₂)=ψ_(RS)(f ₂)+ψ_(HE)(f ₂)+ψ_(ZF3)(f _(ZF2))−2f ₂ D ₃ /c.  (8)

[0065] For the phases of x₄, it follows correspondingly from equation(4):

ψ₃(f ₁)=ψ_(HS)(f ₁)+ψ_(RE)(f ₁)+ψ_(ZF4)(f _(ZF1))−2f ₁2D ₁ /c  (9)

and

ψ₄(f ₂)=ψ_(RS)(f ₂)+ψ_(RE)(f ₂)+ψ_(ZF4)(f _(ZF2))−2f ₂ D ₂ /c.  (10)

[0066] It is particularly advantageous when the modulation frequenciesf₁ and f₂ are exchanged in the next step, so that the main emitter 1 nowemits radiation with modulation frequency f₂ and the reference emitter 2emits radiation with modulation frequency f₁. As will be describedsubsequently, this measurement with exchanged modulation frequenciesimproves the definite distance measurement very substantially. Accordingto the method described above, the following phases are measured:

ψ₁(f ₂)=ψ_(HS)(f ₂)+ψ_(HE)(f ₂)+ψ_(ZF3)(f _(ZF2))−2f ₂2D ₀ /c  (11)

ψ₂(f ₁)=ψ_(RS)(f ₁)+ψ_(HE)(f ₁)+ψ_(ZF3)(f _(ZF1))−2f ₁ D ₃ /c  (12)

ψ₃(f ₂)=ψ_(HS)(f ₂)+ψ_(RE)(f ₂)+ψ_(ZF4)(f _(ZF2))−2f ₂2D ₁ /c  (13)

ψ₄(f ₁)=ψ_(RS)(f ₁)+ψ_(RE)(f ₁)+ψ_(ZF4)(f _(ZF1))−2f ₁ D ₂ /c.  (14)

[0067] By subtraction, it follows from equations (7-14):

ψ₁(f ₁)−ψ₃(f ₁)=ψ_(HE)(f ₁)−ψ_(RE)(f ₁)+ψ_(ZF3)(f _(ZF1))−ψ_(ZF4)(f_(ZF1))−2f ₁2D ₀ /c+2f ₁2D ₁ /c  (15)

ψ₂(f ₂)−ψ₄(f ₂)=ψ_(HE)(f ₂)−ψ_(RE)(f ₂)+ψ_(ZF3)(f _(ZF2))−ψ_(ZF4)(f_(ZF2))−2f ₂ D ₃ /c+2f ₂ D ₂ /c  (16)

ψ₁(f ₂)−ψ₃(f ₂)=ψ_(HE)(f ₂)−ψ_(RE)(f ₂)+ψ_(ZF3)(f _(ZF2))−ψ_(ZF4)(f_(ZF2))−2f ₂2D ₀ /c+2f ₂2D ₁ /c  (17)

ψ₂(f ₁)−ψ₄(f ₁)=ψ_(HE)(f ₁)−ψ_(RE)(f ₁)+ψ_(ZF3)(f _(ZF1))−ψ_(ZF4)(f_(ZF1))−2f ₁ D ₃ /c+2f ₁ D ₂ /c,  (18)

[0068] and, finally, by subtracting equations (15), (18) and (16), (17),it follows:

ψ(f ₁)=2f ₁2D ₁ /c−2f ₁ D ₂ /c+2f ₁ D ₃ /c−2f ₁2D ₀ /c+2n  (19)

ψ(f ₂)=2f ₂2D ₁ /c+2f ₂ D ₂ /c−2f ₂ D ₃ /c+2f ₂2D ₀ /c−2n  (20).

[0069] Since the phase can only be definitely measured in an interval of0 to 2, but the measurement distance in most cases producessubstantially larger phase shifts, the integral number n of the fullcycles which, in addition to the residual phase term, determines thetotal phase rotation is introduced in equations (19) and (20). Thedistance D₀ to be measured and the number of cycles n can now bedefinitely determined from the last two equations (19) and (20), sincethe distances D₁, D₂ and D₃ internal to the device are constant and canbe determined beforehand by measurement technique. The two modulationfrequencies f₁ and f₂ should lie close enough together that the samenumber of cycles n results for both equations (19) and (20). Thisambiguity of the measurement distance is the reason for exchanging themodulation frequencies according to the teaching of patent claim 2,because the additional measurement with exchanged frequencies suppliesthe additional equation (20) which is independent from equation (19).These two independent equations also supply definite values for n and D₀with large measurement distances D₀.

[0070] Another advantage of the additional measurement with exchange offrequencies is that the main receiver phases and reference receiverphases (ψ_(HE)(f₁), (ψ_(HE)(f₂), (ψ_(HE)(f₁), (ψ_(HE)(f₂)) and thephases of the intermediate frequency range can be completely eliminatedas can be seen in equations (17) and (18). This is achieved bysubtracting equations (15) and (18) and equations (16) and (17).

[0071] The phase differences on the respective left-hand side ofequations (19) and (20) result from the phase measurement.

[0072] A smaller frequency difference f₁−f₂ (e.g., several hundreds ofkHz, with f₁=900 MHz) is desired on the one hand in order to definitelydetermine the integral number of cycles n (the same n in equations (19)and (20)) at large measurement distances (e.g., >100 m). On the otherhand, with smaller frequency differences there are greater measurementerrors caused by noise, so that n is incorrectly determined undercertain conditions.

[0073] For more exact measurement of large distances and, at the sametime, accurate determination of the number of cycles n, it isadvantageous to use a second frequency pair f₃ and f₄ for intensitymodulation in a further measurement pass which differs from f₁ and f₂,for example, by 10 MHz. The procedure mentioned above is carried outagain with this frequency pair, wherein no exchange is required becausethe exact distance measurement is carried out with frequency pair f₁ andf₂. Due to the larger frequency difference (e.g., f₃−f₁=10 MHz), apossible measurement error is further reduced and the integral cyclenumber n can also be definitely determined with very large measurementdistances. By means of this step, indicated in patent claim 18, whichconsists in a uniform, small change in the intensity modulationfrequencies, distance and cycle number can be determined without errorswith weak, noisy measurement signals through the use of anotherfrequency pair, e.g., f₁−10 MHz, f₂−10 MHz. In addition, by changing themeasurement frequencies, optimum-operating points can be found whichresult in optimal signal-to-noise ratios. Due to the tolerances ofbandpass filters, these optimal frequencies may differ slightly from onedevice to the other.

[0074] The method according to the invention and the distancemeasurement device based on this method are distinguished, above all, bythe following advantages:

[0075] All phase errors are eliminated by simultaneous measurement ofthe reference emitter signal and the main emitter signal. Therefore, allphase errors dependent upon temperature, aging and reception power inthe transmitting unit and reception unit are completely eliminated.

[0076] Measurement accuracy is substantially improved.

[0077] The reliability of measurements is appreciably improved.

[0078] The measurement device does not require maintenance to a greatextent because no mechanical changeover switches or the like arerequired.

[0079] The measurement time is reduced and the measurement accuracy isincreased by the simultaneous measurement of the reference emittersignal and main emitter signal.

What is claimed is:
 1. A method for optoelectronic distance measurement,wherein an intensity-modulated main light beam emitted by a main lightemitter is directed to a measurement object at a distance, wherein thedistance (D₀) of a measurement object from an observation point is to bemeasured, and a light scattered at the observation point reaches a mainphotoreceiver via reception optics, and a branched part of the mainlight beam is simultaneously directed to a reference photoreceiver via afirst known reference distance (D₁), an intensity-modulated referencelight beam emitted by a reference light emitter reaches the referencephotoreceiver via a second known reference distance (D₂), and a part ofthe reference light beam reaches the main photoreceiver via a thirdreference distance (D₃), and wherein the signals delivered by the mainphotoreceiver and the reference photoreceiver are subjected to acomparative signal evaluation for obtaining a corrected measurementsignal, wherein the light intensities of the main emitter and thereference emitter are simultaneously intensity-modulated at differentfrequencies, wherein the signal mixtures supplied by the main receiverand the reference receiver, each of the signal mixtures contains asignal component with the intensity modulation frequency of the mainemitter and a signal component with the intensity modulation frequencyof the reference receiver, are converted to an intermediate frequencyrange containing two frequency components, wherein one frequencycomponent is formed by the signal of the reference emitter and the otherfrequency component is formed by the signal of the main emitter, and theseparation of the phase information contained in the two simultaneouslyoccurring intermediate frequency signals is based on the differentfrequencies in the intermediate frequency range and the differentmodulation frequencies for the intensity modulation of the main lightbeam and reference light beam for purposes of comparative signalevaluation.
 2. The method of claim 1 , wherein a plurality ofmeasurement processes are successively carried out to achieve a definitedistance measurement and to improve the measurement results, wherein themodulation frequencies for the intensity modulation are exchanged forone another and changed equally according to a set pattern.
 3. Themethod of claim 1 , wherein the delivery power of at least one of themain emitter and the reference emitter is varied for adapting todifferent dynamic requirements.
 4. The method according to claim 1 ,wherein the main light emitter comprises a first laser and the referencelight emitter comprises a second laser.
 5. The method of claim 4 ,wherein the fundamental wavelengths (λ₁, λ₂) for the first and secondlasers are selected differently.
 6. The method of claim 5 , whereincrosstalk between the light signal paths associated with the main beamand reference beam is reduced by optical filtering.
 7. The methodaccording to claim 4 , wherein the phases of the signal components ofthe signal mixtures in the intermediate frequency range are determinedby digital Fourier transformation with evaluation of the real andimaginary parts of the Fourier-transformed signal mixtures in thefrequency domain at the respective intermediate frequencies (f_(ZF1) andf_(ZF2)) for comparative signal evaluation.
 8. The method of claim 7 ,wherein the signal mixtures to be supplied to the digital Fouriertransformation are subjected to a low-pass filtering.
 9. The method ofclaim 8 , wherein the intermediate frequencies (f_(ZF1) and f_(ZF2)) andthe sampling times of an analog-to-digital conversion preceding theFourier transform are selected such that an integral number of cycles ofthe signal component with the first intermediate frequency (f_(ZF1)) andthe signal component with the second intermediate frequency (f_(ZF2))lie in the measurement window of the digital sampling.
 10. The methodaccording to claim 1 , wherein the component of the reference light beamreaching the main receiver is initially diffusely scattered and isguided to the main receiver only as a scattered component together withthe backscattered component of the main light beam impinging via thereception optics.
 11. The method according to claim 1 , wherein thecomponent of the reference light beam reaching the main receiver isinitially diffusely reflected and is guided to the main receiver only asa scattered component together with the backscattered component of themain light beam impinging via the reception optics.
 12. The methodaccording to claim 1 , wherein components of the reference light beamand main light beam reaching the reference receiver are initiallydiffusely scattered and are guided to the reference receiver only as ascattered light components.
 13. The method according to claim 10 ,wherein components of the reference light beam and main light beamreaching the reference receiver are initially diffusely scattered andare guided to the reference receiver only as a scattered lightcomponents.
 14. A device for optoelectronic distance measurement withtwo light transmitters each having a light beam that isintensity-modulated, wherein the light beam of the first lighttransmitter being a main emitter, is directable to a measurement objectat a distance, wherein the distance (D₀) of the measurement object froman observation point is to be measured, and a separate light componentreaches one of two photoreceivers, via a first reference distance (D₁),and wherein the light beam of the second light transmitter, reaches oneof the reference receivers via a second reference distance (D₂), and abeam component separated from the light beam of the second lighttransmitter reaches a second photoreceiver being a main receiver, via athird reference distance (D₃), the second photoreceiver being acted uponin addition by a component of the light beam of the main emitterbackscattered from the measurement object, with two signal mixers eachbeing associated with the reference receiver and the main receiver andtransforming the receiver signal mixtures into an intermediate frequencyrange, and an evaluating device for determining the measurement distance(D₀) from the output signals of the two mixers, wherein a device bywhich the light beams emitted by the main emitter and reference emittercan be simultaneously intensity-modulated with different frequencies(f₁, f₂).
 15. The device of claim 14 , wherein at least one of atransimpedance amplifier and impedance-monitored HF amplifier isarranged between at least one of the reference receiver and the mainreceiver and the respective associated mixer.
 16. The device of claim 14, wherein at least one the main receiver and the reference receiver isan avalanche photodiode.
 17. The device of claim 16 , wherein theavalanche photodiode is a direct mixer, wherein a reception signalmixture is converted directly into the intermediate frequency range bymodulation of the avalanche gain via a local oscillator signal (f₁₀)generated by a local oscillator.
 18. The device of claim 17 , whereinthe local oscillator is an LC oscillator, the oscillation-determiningelements of the LC oscillator being formed by the capacity of theavalanche photodiode.
 19. The device of claim 14 , wherein at least oneof the main receiver and the reference receiver is a PIN photodiode. 20.The device of claim 14 , wherein device is present in the intensitymodulation device for mutual exchange of frequencies (f₁, f₂) of themodulation signals reaching the respective light emitter.
 21. The deviceof claim 20 , wherein the device is used for time-sequential mutualexchange of the frequencies of the intensity modulation signals reachingthe respective light emitter.
 22. The device of claim 20 , wherein thedevice is used for uniformly changing the frequency of the intensitymodulation signals reaching the respective light emitter.
 23. The deviceof claim 14 , wherein a scattering device is arranged in the beam pathof a beam component traveling from the reference emitter to the mainreceiver and wherein a scattered light component of the beam componentproceeding from the reference emitter travels from the scattering deviceto the main receiver.
 24. The device of claim 14 , wherein a scatteringdevice is arranged in the beam path of beam components traveling fromthe reference emitter and from the main emitter to the referencereceiver and wherein scattered light components of the beam componentsproceeding from the reference emitter and main emitter travel from thescattering device to the reference receiver.
 25. The device of claim 14, wherein the device varies the light output of at least one of thereference emitter and the main emitter.
 26. The device of claim 14 ,wherein the optical wavelengths (λ₁, λ₂) of the reference emitter andthe main emitter are different.
 27. The device of claim 26 , wherein atleast one of (i) an optical filter tuned to the wavelength emitted bythe main emitter is arranged in the beam path from the main emitter tothe reference receiver and (ii) an optical filter tuned to thewavelength emitted by the reference emitter is arranged in the beam pathfrom the reference emitter to the main receiver.
 28. The device of claim27 , wherein at least one of the main emitter and the reference emitteris one of an edge emitting laser diode, a VCSEL (Vertical Cavity SurfaceEmitting Laser Diode) and a light diode.
 29. The device of claim 14 ,further comprising a mother oscillator to be used for both thetransmitting part and reception part.
 30. The device of claim 14 orclaim 17 , wherein the two mixers are controllable by the same localoscillator with a frequency (f_(LO)) selected such that there is asignal mixture present in the intermediate frequency range whose signalcomponents with both intermediate frequency components (f_(ZF1),f_(ZF2)) contain phases in the modulation frequencies (f₁, f₂).